Rotary rock bits are well known in the drilling art and typically comprise a bit body having three or more support arms depending therefrom and carrying journal pins which project downwardly and radially inwardly from the support arms. Cutting cones are coaxially supported on the journal pins for rotation relative thereto, and they are captively retained on the pins, by annular arrays of ball bearings disposed within facing annular races. Sealing means maintain lubricant in the bearing areas and prevent entry of borehole fluids and detritus therein. During rotation of the bit body within the earth the cutting cones are caused to rotate relative to their supporting journal pins to thereby perform the cutting function of the bit. Drilling fluid under high pressure is commonly forced through the bit remove cuttings.
A longstanding problem heretofore associated with conventional rock bits of this type is that when a bearing seal fails the inner surface of the cone and the loaded side of the journal pin begin to wear away, thereby progressively widening the gap between the top of the journal and the cone. At a certain point in time the width of such a gap can increase to an extent such that the ball bearings can escape and permit the cone to fall off its journal pin.
Cone loss must be avoided since a cone in the bottom of a drill hole can render further drilling extremely difficult, if not impossible, if the lost cone cannot be successfully fished out. The fishing-out of a separated cone is usually a laborious, time-consuming and expensive endeavor.
Heretofore, the prevention of cone loss is commonly accomplished by estimating the drilling time to which a given rock bit may be exposed, and by carefully monitoring the penetration rate, rotary torque and drill string action during this time.
The accuracy of this time estimate is unavoidably dependent upon a wide variety of factors. If this time estimate is overly conservative, unnecessary drill bit replacement costs may be incurred. On the other hand, if the time estimate is overly optimistic, and bit failure signals are not observed, cone loss can occur.
There have been several attempts to develop devices which give the driller signals that damage has occurred to the bit and it needs to be replaced, as indicated by these Pat. Nos.: 3,062,302; 3,853,184; 2,580,860; 2,647,729; 3,058,532; 3,560,328; 3,982,432; 2,925,251; 3,345,867; 3,363,702; 4,655,300; 4,785,894; 4,785,895; 4,730,681.
From the foregoing it can be seen that it would be highly desirable to provide improved rotary rock bit apparatus and associated drilling methods which eliminate or minimize above mentioned and other limitations and disadvantages typically associated with rock bits of conventional construction. Accordingly, it is an object of the present invention to provide such apparatus and methods.